The injury, which occurred during Tuesday's practice, represents a major setback for a Badgers defense that ranked fourth in points allowed per game last season.
Wisconsin linebacker Jack Cichy, who was expected to be one of the leaders of a stout Badger defense this fall, will miss the 2017 season with a torn ACL. "You only get so many chances to play this game and you never want to see a guy miss any of those opportunities".
Wisconsin coach Paul Chryst added: "We're most disappointed for Jack, but if you know him you know there's a lot to him".
Cichy was set to enter his fourth season with Wisconsin.
Cichy's injury will leave a large gap at the linebacker position, especially with the likes of Vince Biegel and T.J. Watt graduating to the NFL.
Last season, he tore his pectoral muscle in October and was limited to seven games as a result. In his seven starts, Cichy recorded 60 tackles, 7.0 tackles for loss and 1.5 sacks.
The senior linebacker redshirted in 2014, so he has already used up his eligibility, but he's likely to apply, and be granted, a medical redshirt.
The 6-2, 223-pound sophomore was on the Bednarik and Nagurski watch lists in 2017 after earning honorable mention All-Big Ten honors in 2016 and being a Butkus semifinalist.
The Badgers will open up their regular season on Friday, Sept. 1 when they host Utah State.